1. Scope of the Invention
This invention relates principally to Ultra-Wideband radar or data link transmission, and more specifically to the technique for reducing the bandwidth and increasing the radiated energy of these signals by using an array of low-cost transmitters.
2. Description of the Prior Art
The use of Ultra-Wideband (UWB) transmissions in both radar and data communications has been developed both for military and commercial applications. UWB radar systems consist of replacing the conventional pulse modulated or swept cw transmitter with a baseband or impulse-like source which excites a UWB antenna cavity, as shown in G. F. Ross, K. Robbins, "Baseband Radiation and Reception System", U.S. Pat. No. 3,739,392. The antenna radiates, essentially, its impulse response which consists of a transient signal of several RF cycles; the nominal center frequency of this short pulse burst of radiation is determined by the dimensions of the antenna cavity. The signal reflects off a target and returns to a similarly designed receiving antenna where it can be processed by a tunnel diode constant false alarm rate (CFAR) receiver similar to A. M. Nicolson, R. Mara, "A Detector Having a Constant False Alarm Rate", U.S. Pat. No. 3,983,422. In the process of reflecting off a target, the signal is further dispersed by the scattering properties of the target. In the communications mode, the signal does not have the benefit of a synchronizing cable between the transmitter and receiver and different schemes must be developed to "lock" the received signal to the source clock. For reference, see G. F. Ross, "Short Baseband Pulse Communications System", U.S. Pat. No. 3,728,632. Here, the range of the system is much greater because of only one-way transmission. And the signal is not dispersed by the target.
These systems transmit very short duration signals, for example, nanosecond or subnanosecond durations. For this reason, very little energy is radiated. Although this radiation for systems having a peak power of 1 kw or less is not likely to interfere with narrowband receivers, these systems do spread their available spectral energy over a very wide band (hence the UWB) and can place energy in restricted FCC bands. These signals, however, are, generally, substantially below kTB-NF for any given receiving channel, where: k is Boltzmans constant; T, the temperature; B, the bandwidth; and NF, the noise figure.
Because of FCC regulations, it may be necessary to reduce the bandwidth of a UWB transmission so that its spectral energy does not extend beyond assigned limits. An inefficient method to reduce the bandwidth of a radiated UWB signal is to "disperse" the pulse by using a narrowband antenna. For example, instead of a transmission bandwidth BW, the bandwidth may be reduced to bw by the selection of an appropriate antenna. Now, the total energy radiated is reduced directly by the ratio ##EQU1## . Consider the pulse modulated sinusoidal signal shown in FIG. 3a. It follows from linear system theory that, in the time domain, the waveform still resonates at f.sub.o, but the signal becomes more dispersive and the amplitude decreases. The duration of the signal increases directly as while the voltage amplitude of the signal decreases as . Thus, the energy E.sub.2 in the new transmission is decreased by: i.e., by the direct reduction of the bandwidth. This follows from Parseval's theorem, which states that all the given energy in the time domain is spread over the entire frequency band. Mathematically, this is given as: ##EQU2## where ##EQU3## are Fourier transforms. The important observation to make here is that the amplitude of the radiated signal has decreased directly by the required bandwidth reduction. One may be required to reduce signal bandwidth further to meet certain FCC band limitations; the result of reduced signal bandwidth is the reception of a much smaller and dispersive signal for a given input energy level. The pulse modulated signal shown in FIG. 3a, f(t) and its Fourier transform, F(.omega.), are given by: F(.omega.) is shown in FIG. 3b. The envelope function [U(t)-U(t-T)] and its spectrum are shown in FIG. 3b.
As first indicated, the approach of narrowing the bandwidth of a UWB transmission to attempt to accommodate FCC regulation, as described above, is inefficient. Generally, one attempts, via UWB technology, to generate a large video or impulsive-like waveform in the most economical manner, excite an antenna cavity, and radiate the resulting pulse with minimum distortion. A time differentiation, due to the radiation properties of the antenna, is a necessary degree of distortion (dispersion) that follows from Maxwell's equations. The one or two additional cycles in the radiated waveform due to antenna distortion can be minimized by resistive loading (e.g., a further energy loss) or by using more optimum antennas (e.g., certain wire antennas, TEM mode waveguide horns, etc.). The use of a TEM mode horn antenna minimizes distortion, but the net energy radiated at any given angle is less; that is, the energy is radiated over much wider angles.
Accordingly, it is an object of the invention to provide a technique to decrease the required bandwidth for FCC purposes while achieving higher radiated energy and, at the same time, taking full advantage of the low-cost techniques inherent in the generation of UWB signals by impulsing antenna cavities. These two objectives may apppear contradictory. However, while the economical approach of generating RF signals by impulse excitation of antennas was developed for UWB systems, it is also applicable to narrowband, high-energy transmissions, as will be shown below.